Heretofore, sound motion pictures have employed three principal types of sound tracks; namely, variable density optical tracks, variable area optical tracks, and magnetically-recorded tracks. The present invention is related to the first two types; namely, optical sound tracks which are intended to be synchronously "read" in conjunction with the projection of the related image frames on the film in a conventional and well-known manner.
Heretofore, motion picture projectors intended for use with sound motion picture film of the type having optical sound tracks, whether of the variable area type or the variable density type, have employed a continuously energized "exciter" lamp which illuminated the sound track on the film with a narrow beam having a uniform light flux. This beam was of roughly rectangular shape, having a transverse dimension coextensive with the width of the sound track, and a height which was less than the equivalent wavelength of the highest audio frequency to be reproduced. This beam was focused onto a photoelectric cell located on the side of the sound track opposite the exciter lamp.
IN PRACTICE, THE EXCITER LAMP WAS ENERGIZED WITH DIRECT CURRENT SO AS TO PROVIDE A STEADY LIGHT FLUX. Variations in the optical transmission of the sound track, in consonance with the recorded audio signal, modulated the light flux falling on the photocell, thereby generating an audio output signal. A very narrow slit in the light path from the exciter lamp constrained the light to a beam of proper dimension for illuminating or "exciting" the moving sound track on the film. This structure and its appurtenances are collectively called the "light gate" and with the addition of the exciter lamp and photocell are collectively known as the "sound reading head".
While sound heads of the prior art have been commercially satisfactory, they have suffered from such shortcomings as the requirements for scrupulous cleanliness, constant maintenance, and less than optimal signal-to-noise ratio. Furthermore, sound heads of the prior art require a high degree of shielding from extraneous ambient light, lest such light degrade the desired output audio signal.
Other limitations of prior art sound reading heads are that they utilize incandescent lamps whose filaments are subject to mechanical vibration of the filament; also they are subject to variations in the power supply, and they have a relatively short operational life. These and other shortcomings of prior art devices are overcome by the novel and improved apparatus and method of the present invention.